Computed Tomography (CT) scanners of the third generation type include an X-ray source and an X-ray detector system secured respectively to diametrically opposite sides of an annular-shaped disk. The disk is rotatably mounted within a gantry support so that during a scan, the disk continuously rotates about a rotation axis, commonly referred to as the "Z-axis", while X-rays pass from the source through an object positioned within the opening of the disk to the detector system. The center of the disk, which is intersected by the Z-axis, is commonly referred to as the "isocenter".
The detector system typically includes an array of detectors disposed as a single row in the shape of an arc having a center of curvature at the point, referred to as the "focal spot", where the radiation emanates from the X-ray source. The X-ray source and the array of detectors are positioned so that the X-ray paths between the source and each detector all lie in the same plane (hereinafter the "slice plane " or "scanning plane") which is normal to the rotation axis of the disk. Since the X-ray paths originate from what is substantially a point source and extend at different angles to the detectors, the X-ray paths resemble a fan, and thus the term "fan beam" is frequently used to describe all of the X-ray paths at any one instant of time.
The X-rays incident on a single detector at a measuring instant during a scan are commonly referred to as a "ray", and each detector generates an output signal indicative of the intensity of its corresponding ray. Since each ray is partially attenuated by all the mass in its path, the output signal generated by each detector is representative of the density of all the mass disposed between that detector and the X-ray source (i.e., the density of the mass lying in the detector's corresponding ray path). The output signals generated by the X-ray detectors are generally filtered by a data acquisition system (DAS) to, among other things, improve their signal-to-noise ratio, and the output signals generated by the DAS are commonly referred to as "raw data signals". The raw data signals are normally filtered by a projection filter which converts the raw data signals to projection data signals by logarithmically processing the raw data signals so that each projection data signal is representative of the density of the mass lying in a corresponding ray path. The collection of all the projection data signals at a measuring instant is commonly referred to as a "projection" or a "view". During a single scan, as the disk rotates, a plurality of projections are generated such that each projection is generated at a different angular position of the disk. The angular orientation of the disk corresponding to a particular projection is referred to as the "projection angle".
Using well known algorithms, such as the Radon algorithm, a CT image may be generated from all the projection data signals collected at each of the projection angles. A CT image is representative of the density of a two dimensional "slice", along the scanning plane, of the object being scanned. The process of generating a CT image from the projection data signals is commonly referred to as "filtered back projection" or "reconstruction", since the CT image may be thought of as being reconstructed from the projection data.
CT scanners generally use some form of "beam hardening filter" to "harden" the X-ray beam generated by the X-ray source. The beam hardening filter is generally implemented as a sheet of metal, such as copper or aluminum, disposed between the X-ray source and the patient. Each X-ray photon generated by the X-ray source has a probability of passing through the filter, and this probability increases as the energy of the photon increases. So the filter tends to intercept the lower energy (or "softer") X-rays and thus "hardens" the beam. In the field of CT scanners it is generally believed that it is preferable to use a hard beam because given a certain level of hard X-rays and the same level of soft X-rays, a greater proportion of the soft X-rays have a higher probability of being absorbed by the human body than the corresponding hard X-rays, and thus are less likely to pass through the patient and reach the detectors. In other words, it is currently believed that the use of soft X-rays increases the dose of radiation to which the patient is exposed without significantly contributing to the generation of the CT image. The United States Food and Drug Administration (FDA) has established minimum beam hardness standards CT scanners that operate on human patients must meet.
Further, increasing beam hardness tends to increase the linearity of the X-ray detectors. Accordingly, most prior art CT scanners use a relatively hard beam to improve the linearity of the detectors and thereby simplify the design of the scanner. In fact, most prior art CT scanners use a relatively high power (e.g., 120 kV at 100 to 350 mA) X-ray source and also use a relatively thick (e.g., 0.008 inches of copper) beam hardening filter which hardens the beam by significantly more than is required by the FDA. The combination of a high power X-ray source and a thick beam hardening filter simplifies the design of the CT scanner, however, it is wasteful of X-ray power and increases the overall power requirements of the scanner.
Another factor tending to increase the X-ray power used by prior art CT scanners relates to the distance between the focal spot and the isocenter of the disk. Since the intensity of an X-ray beam decreases with the square of the distance from the X-ray source, the power used in the X-ray source is determined in part by the distance between the focal spot and the isocenter. (This factor is generally described in terms of the distance between the focal spot and the isocenter rather than in terms of the distance between the focal spot and the detectors because during a scan the patient is generally located at or near the isocenter, and while it is generally possible to increase the X-ray energy received by a detector by increasing the size of the detector, it is only possible to increase the X-ray energy at the isocenter (i.e., at the patient) by increasing the power of the X-ray source or by decreasing the distance between the focal spot and the isocenter.)
Designing a CT scanner for reduced X-ray power consumption therefore suggests placing the isocenter as close as is possible to the X-ray source. However, other competing design criteria require increasing the spacing between the X-ray source and the isocenter. For example, at a minimum, the isocenter must be spaced apart from the X-ray source at least far enough to allow a patient to be comfortably positioned between the X-ray source and the detectors. A phenomenon generally known as "Z-axis beam shifting" places an even greater requirement on the spacing between the X-ray source and the isocenter. Beam shifting relates to movement of the focal spot (i.e., the point from which the X-rays emanate) relative to the isocenter (or the detectors) during a scan. The generation of X-rays produces localized heat which can cause thermal expansion of components of the X-ray source, and this thermal expansion can in turn cause the focal spot to shift relative to the isocenter during a scan. This shift is generally referred to as Z-axis beam shifting because most X-ray sources are configured so that the majority, if not all, of the shift occurs in a direction that is parallel to the Z-axis (i.e., the rotation axis of the scanner disk). Since the Radon algorithm assumes that the relative positions of the X-ray source and the detectors remain constant during a scan, it is important to minimize the effects of any Z-axis beam shifting. In prior art CT scanners, the most common method of minimizing this effect is to increase the spacing between the X-ray source and the collimator used to form and shape the beam, and thereby reduce the angular shift between the focal spot and the detectors caused by any Z-axis translation of the focal spot. Lengthening the distance between the focal spot and the collimator necessarily increases the distance between the focal spot and the isocenter since a certain amount of space is required to receive a patient. In the field of CT scanners, it is generally believed that if the focal spot is not spaced at least 510 mm apart from the isocenter of the disk, the collimator must be placed too closely to the focal spot such that errors caused by Z-axis beam shifting will become too severe. However as stated above, any increase in this distance necessitates a corresponding increase in the power applied to the X-ray source. Prior art CT scanners which increase this spacing therefore do so at the cost of increasing their X-ray power requirements. Thus, such prior art scanners expose the patient to higher levels of X-radiation, since increasing the spacing between the source and isocenter to accommodate a greater spacing between the focal spot and a collimator necessitates generating higher energy X-rays.
In addition to reducing the distance between the X-ray source and the isocenter, X-ray power consumption can be reduced by using high efficiency detectors, such as solid state detectors (e.g., cadmium tungstate detectors). However, many prior art CT scanners use less efficient, gas tube detectors, such as Xenon (Xe) detectors, because the response of such detectors tends to be more uniform than the response of high efficiency detectors. In particular, such low efficiency detectors detect photons more uniformly in the Z-axis direction than do high efficiency detectors. Therefore, prior art CT scanners which use high efficiency detectors also use a large spacing between the focal spot and the collimator (and as a result a large spacing between the focal spot and the isocenter) so as to minimize the effect of any Z-axis beam shifting, resulting in a sacrifice of much of the power savings achieved by the high efficiency detectors. Similarly, many prior art CT scanners which use reduced spacing between the focal spot and the collimator and thus a reduction in the spacing between the focal spot and the isocenter, at the 510 mm spacing for example, also use low efficiency detectors to minimize the effect of any Z-axis beam shifting, and thereby sacrifice much of the power savings achieved by the reduced geometry.
As was stated above, prior art CT scanners also normally use a DAS to, among other things, improve the signal-to-noise ratio of the output signals generated by the X-ray detectors. However, prior art DASs are often implemented using an integrating filter which does not significantly improve the signal-to-noise ratio of the detector output signals. Therefore, prior art scanners typically use high intensity X-ray beams to insure that the signal-to-noise ratio of the detector output signals is sufficiently high to guarantee generation of accurate CT images.
In general, prior art CT scanners have been designed to improve the accuracy of the generated CT images at the expense of requiring increased X-ray power levels. There is therefore a need for a CT scanner which generates high quality CT images and also has reduced X-ray power requirements. There is also a need for a CT scanner which generates high quality CT images while exposing the patient to lower levels of radiation.